Silicon nitride and sialon based materials are recognized as ceramics in high temperature engineering applications due to their high chemical and thermal stability combined with superior mechanical properties. In these systems, self-diffusion processes are relatively slow and they may require an oxide-sintering additive to provide conditions for liquid phase sintering. During sintering, the additive reacts with silica on the surface of the silicon nitride particles and some of the nitride forms an oxynitride liquid, which is converted into a glass phase during cooling. The composition and volume fraction of such oxynitride glass phases strongly influence the properties of the materials.
The glass forming regions have been investigated by many different researchers. The existence of sialon glass was first reported by K. H. Jack, J. Mat. Sci. Rev. 11 (1976) 1135-1158. T. H. Elmer, and M. E. Nordberg, J. Amer. Cer. Soc. 50 (1967) 275-279, introduced nitrogen into a high silica glass by heating the melt in NH3 atmosphere. H. O. Mulfinger, J. Amer. Cer. Soc. 49 (1966) 462-467 produced nitrogen containing soda-lime glass by adding Si3N4 to the synthesis mixture. The effect of nitrogen concentration in silica glass on the hardness properties was studied by Shillito et al. Cer. Soc. 63 (1978) 537. It was suggested that the incorporation of N2 into a silica glass affects the material properties due to the difference in the bonding strength of Si—N compared to Si—O.
By comparing Y-sialon glasses with SiO2—Y2O3—Al2O3 glasses an increase in hardness could be pointed out by introducing nitrogen in the silica glass, where oxygen atoms are partly replaced by N3−. The hardness of the glasses was increased with an increase of nitrogen content.
R. E. Loehman, J. Non-Crys. Solids 56 (1983) 123-134 disclosed that mixtures of oxides and nitrides could be melted and quenched to form glass. By introducing nitrogen into the oxosilicate glass, several material properties were improved, such as increase in the glass transition temperature, hardness, fracture toughness, elastic modulus and chemical durability.
The dissolution of nitrogen in oxosilicate melts was further studied by E. A. Dancy and D. Janssen, Canadian Metallurgical Quarter 15[2] (1976) 103-110, who reacted CaO—Al2O3—SiO2 at 1550° C. in 1 atm. N2 gas. The amount of 0.25 to 2.5 wt % nitrogen could be incorporated by this technique while as high as 4 wt % nitrogen was incorporated by dissolving solid Si3N4 in the melt. The nitrogen concentration in the melt is probably due to the strong and much favorable triple bond in the N2 molecule.
Jack et al. disclosed bulk samples of oxynitride glass obtained by pressure-less heat treatment of a mixture of 14Y2O3-59SiO2-27AlN in a boron nitride crucible at 1700° C. in nitrogen atmosphere. This sample was found to have a refractive index of 1.76 and a nitrogen concentration of 9 at % corresponding to an O:N ratio of 86:14.
Silicate glass is usually made from oxosilicates. The highest possible condensation degree in pure oxosilicates is found for SiO2, wherein every oxygen atom is coordinated by two silicon atoms. It is possible to form glass from pure SiO2.
This form of glass has been found to have many superior physical properties, such as a high melting point, good mechanical properties and transparency for UV photons. However, a high synthesis temperature is needed for the formation of SiO2 glass. Glass modifiers such as Na+, K+ and Ba2+ are added to SiO2 in different concentrations in order to lower the melting temperatures and the manufacturing cost. By introducing glass modifiers, the network structure of SiO2 is partially broken and some of the oxygen atoms are therefore connected only to one silicon atom. Oxygen atoms connected to only one silicon atom are called apex atoms and oxygen atoms connected to two silicon atoms are named bridging atoms. The three dimensional Si—O network in the glass can be maintained when only one out of four oxygen atoms of the SiO4 tetrahedra are apex. At least three oxygen atoms must be bridging between two silicon atoms to get a three dimensional network.
This restriction of the condensation degree makes it possible to form oxosilicate glass only in the composition range SiO2-MxSiO2.5. The highest concentration of the glass modifier can therefore only be x=1.0 for monovalent cations such as Na+ and K+, x=0.5 for divalent cations such as Ba2+ and Pb2+, x=0.333 for trivalent cations such as La3+ and Y3+ and x=0.25 for the four valent Th4+.
The concept of introducing nitrogen into the glass chemistry has previously been used in sialon glasses. By quenching melts of M-Si—Al—O—N from high temperatures, glass phases of sialons with glass modifiers such as La3+ and Y3+ were obtained. The composition limit concerning Ln (lanthanide) content, which were used as the glass modifiers, and nitrogen content was reached with the composition La5Si10Al5O27.5N5, described by N. K. Schneider, H. Lemercier and S. Hampshire, Materials Science Forum, 325-326 (2000) 265-270. This composition gives the highest lanthanum and nitrogen content ever obtained in a nitride based glass at ambient pressure. The cationic composition given in atomic percent is then La:25%, Si:50% and Al:25% and the anionic composition given in the same way is O:84.2% and N:15.8%. The synthesis technique used for preparation of such glasses has limited the nitrogen content as well as the glass modifier content (lanthanum in the example mentioned above).
Accordingly, the glass materials that are present today have a nitrogen content corresponding to the O:N ratio of 84.2:15.8. However, since demands for new glass materials having higher strength and improved physical properties in other respects, not least for various optical, ceramical and coating-technological applications, continuously are raised, it would be a great advantage to provide new materials with even better properties.
One oxonitride glass with higher lanthanum and nitrogen content has been disclosed by A. Makishima, M. Mitomo, H. Tanaka, N. Ii and M. Tsutsumi, Yogyo-Kyokai-Shi 88[11] (1980) 701, possible to synthesis only at high nitrogen pressure (30 atm.). The composition of this glass have been reported as La19.3Si20.0O42.5N18.2, corresponding to a La:Si ratio of 49:51 and an O:N ratio of 70:30.
W. Schnick et al. Chem., 9 (1999) 289 introduced a route for introducing nitrogen into the silicate chemistry other than the obvious reaction of the silicate melt with N2 gas for synthesis of crystalline nitridosilicates, oxonitridosilicates and oxonitridoaluminosilicates, i.e. not glass materials, by using electropositive metals together with silicon diimide (Si(NH)2) in a radio frequency furnace. The above mentioned synthesis route was accordingly used only for producing crystalline phases.
The glass materials described above have certain limitations in chemical composition regarding both nitrogen content as well as concentration of glass modifiers. The chemical composition of such material is a crucial parameter defining the physical properties and for that reason also different possibilities in applications.
A problem with nitrogen containing glass today is that there are requirements for even better physical properties of glass than is known today. There are no known methods for increasing the nitrogen content of the glass and thereby try to improve its properties. The method of Makishima et al has yielded the highest known nitrogen-content, but that method has the disadvantage of requiring complicated equipment and is expensive.